1. Field of the Invention
The present invention relates generally to methods and apparatuses for forming downhole pressure plugs in a wellbore. More particularly, the present invention relates to methods of forming downhole plugs to seal the wellbore and to transfer stress from a wellbore tool to the wellbore itself.
2. Description of the Prior Art
It is conventional in the oil and gas industry to seal wellbores using packers, bridge plugs, and the like. Typically, a wellbore tool, such as a packer or bridge plug, is run into the wellbore to a desired location therein. The packer or bridge plug is inflated or otherwise actuated into sealing engagement with the wellbore. Such a seal may be effected to separate regions in the wellbore, to contain fluid pressure either above or below the wellbore tool for fracturing or other well treatment operations, or other conventional reasons.
Conventional wellbore tools have a force threshold beyond which the wellbore tool will fail mechanically, or will lose gripping and sealing engagement with the wellbore, which tends to cause undesirable movement of the wellbore tool within the wellbore. The force threshold typically is defined in terms of a maximum or limiting differential pressure across the wellbore tool that the wellbore tool can withstand without failure or movement in the wellbore.
If the force threshold is exceeded, mechanical failure of the wellbore tool or undesirable movement of the wellbore tool may result. Mechanical failure may result in at least partial inoperability of the wellbore tool. If the wellbore tool is rendered inoperable, the wellbore may be undesirably obstructed, requiring expensive fishing remedial operations. Mechanical failure at least will require expensive and time-consuming repair or replacement of the wellbore tool.
Even if the wellbore tool does not fail and is not otherwise damaged, the wellbore tool may be moved or displaced within the wellbore if the force threshold is exceeded. Such movement or displacement is undesirable because the positioning of the wellbore tool within the wellbore frequently is of great importance. Also, movement or displacement of the wellbore tool could damage other wellbore tools or the producing formation itself, thereby necessitating fishing, workover, or other remedial wellbore operations.
In secondary recovery operations, such as formation fracturing, reliable and dependable packers and bridge plugs frequently are necessary. Many secondary recovery operations require sealing off or packing a selected formation interval, and introducing extremely high pressure fluids into the selected interval. High-pressure fluids exert extreme axial forces on the packers or bridge plugs used to seal off the interval. Thus, the possibility of exceeding the force threshold of such wellbore tools is very great in formation fracturing, and requires the use of expensive, reinforced, high-pressure rated wellbore tools. High-pressure wellbore tools typically have relatively large cross-sectional diameters, precluding their use in through-tubing operations or operations in otherwise reduced-diameter or obstructed wellbores.
An alternative to high-pressure rated wellbore tools is to plug or seal the wellbore with cement. Cement plugs have a number of drawbacks. Expensive and specialized cementing equipment usually is required to pump cement into the wellbore to form a cement plug. Also, a significant time period must elapse to permit a cement plug to harden or set into a sealing or load-bearing cement plug. Another drawback of cement plugs is that they are relatively permanent, and require expensive and time-consuming milling operations to remove them from the wellbore.